The present invention relates to methods for forming a capacitor in an integrated circuit ("IC").
Large value or high voltage capacitors are sometimes required in integrated circuits. The creation of a high voltage capacitor necessitates the devotion of a proportionately large circuit area to the capacitor. Required capacitor area may be especially large if high voltage MOS type capacitors are used because the thickness of the capacitor dielectric must be increased to support the high voltage. Since capacitance is inversely proportional to dielectric thickness, increases in the dielectric thickness result in an increase of the dielectric area required to achieve a given capacitance. Examples of such high voltage capacitors include compensation capacitors for operational amplifiers and filter capacitors in low pass filters.
As an illustration, a typical capacitor required in an integrated circuit might be a 100 pf, 200 V capacitor having a dielectric of silicon dioxide. If the dielectric thickness is set at 0.6667 microns to limit the electric field across the dielectric to 3,000,000 V/cm, then the required capacitor area is 0.0194 square centimeters, a relatively large area in an integrated circuit environment. The area required for such a capacitor results in an enlargement of the overall IC containing both the capacitor and the IC device.
In order to conserve space in integrated circuits, capacitors may be formed from the material which is not devoted to the formation of IC devices. Additionally, the cost of manufacturing IC's which include capacitors may be reduced by the use of the same process steps required to make the IC devices in the making of the capacitors.
Accordingly, it is an object of the present invention to provide a novel method of forming a capacitor from material in an integrated circuit which is not used for IC devices.
It is a further object of the present invention to provide a novel method of forming a capacitor in an integrated circuit using some of the same process steps that are used to form devices in the integrated circuit.
It is still a further object of the present invention to provide a novel method of forming a capacitor in an integrated circuit which reduces the risk of shorting between capacitor plates.
It is sometimes desirable to add other circuit structures to the circuit and such structures generally add significantly to the area required for the circuit. It accordingly an object of the present invention to provide a novel method of forming resistive circuit elements on the same area with capacitors.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.